Recently, in the optical pickup apparatus, the wave-length-shortening of the laser light source used as the light source for reproducing of the information recorded in an optical disc or recording of the information in the optical disc is advanced. For example, a laser light source of wavelength 405 nm such as a blue-violet semiconductor laser, or a blue-violet SHG laser which conducts the wavelength conversion of the infrared semiconductor laser by using the second harmonic wave generation is putting to a practical use.
Using these blue-violet laser light sources enables an objective lens with the same numerical aperture (NA) as DVD (Digital Versatile Disc) to record the information of 15-20 GB in an optical disc of diameter 12 cm, and enables an objective lens whose NA is increased to 0.85 to record the information of 23-25 GB in the optical disc of diameter 12 cm. Hereinafter, in the present specification, the optical disc and photo-magnetic disc for which the blue-violet laser light source is used, are generally referred as “high density optical disc”.
Hereupon, as the high density optical disc, presently, 2 standards are proposed. One of them is the Blu-ray disc (hereinafter, BD) which uses the objective lens of NA 0.85, and whose protective layer thickness is 0.1 mm, and another one is the HD DVD (hereinafter, HD) which uses the objective lens of NA 0.65 to 0.67, and whose protective layer thickness is 0.6 mm. When referring to the possibility that, in future, the high density optical discs of these 2 standards are distributed in the market, a high density optical disc player and/or recorder which enables to record and/or reproduce on any high density optical disc similarly, is desired.
Each of the following Patent Documents 1 and 2 shows 2-focal objective lens which can compatibly record and/or reproduce information on 2 kinds of optical discs whose protective layer thickness are different from each other, and light source wavelengths are the same.
In the 2-focal objective lens disclosed in 2 Patent Documents, when a large amount of light amount of the incident light flux is distributed to 2 focus by the diffractive structure formed on the lens surface, the recording and/or reproducing of the optical discs whose protective layer thickness are different from each other is conducted.
[Patent Document 1]Tokkaihei No. 9-179020[Patent Document 2]Tokkaihei No. 9-120027
However, the above 2-focal objective lens is designed on the assumption that information is to be recorded and/or reproduced on the optical disc such DVD whose NA is 0.6 and Compact Disc (hereinafter, CD) whose NA is 0.45 is an subject of the information recording and/or reproducing, and it is difficult that the above 2-focal objective lens records and/or reproduces information on each of BD and HD, whose NA is large.